1. Field of the Invention
This invention relates to a process for recovering petroleum from tar sands by use of a solvent.
2. Description of the Prior Art
Throughout the world there are various known locations wherein the earth contains large deposits of tar sands. For example, one of the most extensive and best known deposits of this type occurs in the Athabasca district of Alberta, Canada. In the tar sands, the oil typically has a density approaching or even greater than that of water. The Athabasca tar sands extend for many miles and occur in varying thicknesses up to more than 200 feet. In many places, the Athabasca tar sands are disposed practically on the surface of the earth. These areas lend themselves to open pit mining operations. The oil content ranges between about 10 and 20 percent by weight; although sands with lesser or greater amounts of oil content are not unusual. Additionally, the sands generally contain small amounts of water in the range from about one to ten percent by weight.
The oil present and recoverable from tar sands is usually a rather viscous material ranging in specific gravity from slightly below 1.00 to about 1.04 or somewhat greater. At a typical reservoir temperature of about 48.degree. F, this oil is immobile, having a viscosity exceeding several thousand centipoise. At higher temperatures such as those above 200.degree. F, or diluted with a suitable solvent, the oil becomes mobile with viscosities of less than about 343 centipoise. Since this tarry material does not generally command a very high price, particularly in its crude state, its separation and recovery must involve a minimum of expenditure in order to economically attractive for commercial practice.
Surface mining techniques for tar sands are well known. Two approaches are generally followed. In the first, a few mining units of custom design are used. These are generally larger units and may be bucket wheel excavators, dredges (both hydraulic and bucket lifter), and super-sized draglines. The second general mining technique is to use many smaller mining units of conventional design. For example, scrapers and truck and shovel operations are typical. Once the tar sand has been recovered by these methods, a ball mill or other similar device may be used to pulverize the tar sand into small pieces suitable for the various recovery techniques. One such recovery technique is solvent extraction. The solvent extractions heretofore attempted, however, have been done at temperatures above the freezing point of the water in the tar sands. As the solvent dilutes the bitumen during these operations, the bitumen and the liquid water form tight emulsions which become a very difficult problem in processing since they are not easily broken. These emulsion problems are well known and are a leading cause of the economic failure of some solvent processes. (See Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 19, page 706).
This invention discloses a way to solve the emulsion problem by preventing the water contained in the tar sands from forming emulsions with the bitumen.